This invention relates to a motor vehicle electrical system and more particularly to a battery state-of-charge maintenance method and system effective during periods of vehicle operation to maintain the battery state-of-charge in accordance with actual vehicle electrical system requirements.
A typical motor vehicle electrical system has a voltage regulated alternator drivingly connected to the vehicle engine to provide power for vehicle electrical loads such as lighting, instrumentation, climate controls and other power accessories during periods of vehicle operation. Ideally, sufficient generated electrical power remains available during all periods of vehicle operation to maintain the battery state-of-charge. However, it is well recognized that there are periods of vehicle operation during which battery state-of-charge is actually reduced due to net vehicle load power consumption exceeding that generated by the alternator.
These periods of operation wherein the battery experiences a reduced state-of-charge may be short in duration with insufficient high engine speed time or may simply be sustained low engine speed periods, both of which may result in a net electrical power draw from the battery. Battery discharge is exacerbated during such periods when the electrical load is relatively high such as is typically experienced during inclement weather when accessories such as rear defogger, heated windshield, heated seats, windshield wipers, headlights, etc. are active. It is also well known that cold temperature reduces the charge acceptance rate of a battery significantly, thereby requiring even longer periods of high engine speed operation. Additionally, a battery which is at least partially discharged and immediately recharged is characterized by a much greater rate of charge acceptance than an equivalently discharged battery which is allowed to sit for a period of 16 hours--a period of time which is roughly equivalent to what is known in the automotive field as an overnight cold soak representative of actual expected periods of overnight inactivity.
Automotive electrical loads continue to grow as customer demand for more creature comfort options such as heated seats, heated windshields, etc. continues to rise. Expansion of electronic controls on powertrain and suspension systems, evolution of navigation, intelligent highway and obstacle detection systems as well as numerous other unmentioned accessories clearly indicate increased demands for electric power in an automobile. Additionally, rising parasitic loads during periods of vehicle in operation make maintaining a high battery state-of-charge under all driving conditions very desirable.
It is well known that alternator output increases with increased engine rpm and most alternators can supply power needed to meet most electrical loads at high speeds. Low engine speed alternator rpm is limited by the drive pulley ratio which in turn is dictated by the maximum allowable alternator rpm at maximum engine speed. The demand for electrical power at idle is fast approaching or exceeds the supply available from the alternator.
Numerous attempts have been made at addressing the various hurdles associated with delivering sufficient electrical power especially at vehicle idle. Some gains have been made by adjusting alternator output in accordance with battery temperature in order to most efficiently recharge the battery. Demand-side electrical load management is incorporated in the design and development of motor vehicles and includes such things as pulse width modulation control of DC motors, exterior styling compromises to minimize bulb usage, alternative lighting schemes such as discharge type beam headlamps, central source lighting and LED brake lamps. Battery rundown protection systems also have been proposed which are intended to monitor parasitic loads and battery state-of-charge during periods of vehicle inoperative to disconnect the battery from vehicle electrical loads prior to the point at which the battery becomes too discharged to restart the vehicle.